Bernie Masters is a geologist/zoologist who spent 8 years as a member of the Western Australian Parliament. Married to Carolina since 1976 and living in south west WA, Bernie is involved in many community groups. This blog offers insights into politics, the environment and other issues that annoy or interest him. For something completely different, visit www.fiatechnology.com.au for information about vegetated floating islands - the natural way to improve water quality.

Friday, May 27, 2016

"the warming effects of carbon dioxide... might have been overestimated"

Cloud-seeding surprise could improve climate predictions

A molecule made by trees can seed clouds, suggesting that pre-industrial skies were less sunny than thought.

Molecules released by trees can seed clouds, two experiments have revealed. The findings, published on 25 May in Nature and Science,
run contrary to an assumption that the pollutant sulphuric acid is
required for a certain type of cloud formation — and suggest that
climate predictions may have underestimated the role that clouds had in
shaping the pre-industrial climate.

If the
results of the experiments hold up, predictions of future climate change
should take them into account, says Reto Knutti, a climate modeller at
the Swiss Federal Institute of Technology (ETH Zurich). For 20 or more
years, clouds have been the largest source of uncertainty in
understanding how manmade emissions affect the atmosphere, he says.

In addition to releasing carbon
dioxide, burning fossil fuels indirectly produces sulphuric acid, which
is known to seed clouds. So, climate scientists have assumed that since
pre-industrial times, there has been a large increase in cloud cover, which is thought to have an overall cooling effect by reflecting sunlight back into space.
And they have assumed that this overall cooling effect has partially
masked the climate’s underlying sensitivity to rising carbon dioxide
levels.

The
latest experiments suggest that it may have been cloudier in
pre-industrial times than previously thought. If this is so, then the
masking effect, and in turn the warming effects of carbon dioxide, might
have been overestimated, says Jasper Kirkby, a physicist at the CERN,
Europe’s particle-physics laboratory near Geneva, Switzerland, who led
one of the experiments.

But Kirkby adds that itis
too early to say whether this is true in practice, or by how much,
because there are so many factors that play into such projections.“There are many uncertainties; we are only talking about one,” says Kirkby. Knutti says the results will probably not affect the most likely projections of warming, as laid out by the Intergovernmental Panel on Climate Change. “Our best estimate is probably still the same," he says.

Aerosols needed

Clouds
are made of microscopic droplets of liquid water or, in some cases, of
small ice crystals. But in the atmosphere, water vapour cannot simply
turn into a cloud: it needs solid or liquid particles, known as
aerosols, on which to condense.
About half of
these aerosols originate, already in solid form, from Earth’s surface:
for instance, dust from deserts, salt crystals from the oceans or soot
from combustion. The other half forms anew in the atmosphere from
gaseous impurities. The individual gas molecules capture more molecules
from the air to form solid particles. If they grow to 50–100 nanometres,
water vapour can condense on them.

Until recently, atmospheric scientists
thought that only sulphuric acid vapour, which can be produced by
volcanic emissions or by burning fossil fuels, could trigger this
process. As a result, it was thought that pre-industrial skies were
somewhat less cloudy than present ones because they contained less of
this pollutant, says Kirkby.

To investigate
the process, he turned to the Cosmics Leaving Outdoor Droplets (CLOUD)
experiment, which he founded. A three-metre tall stainless steel tank
that can reproduce a vast range of atmospheric conditions, CLOUD can be
hooked up to the beams of particles that feed the Large Hadron Collider
(LHC) at CERN. This simulates the effects of cosmic rays — high-energy
subatomic particles that come from outside the Solar System and are
thought to have a role in cloud formation — in the atmosphere.

Cosmic-ray starter kit

In the two Nature papers,
Kirkby and his co-authors report that aerosols can form and grow to the
size needed to seed a cloud from compounds emitted by trees — without
any sulphuric acid and accelerated by simulated cosmic rays. In these
experiments, the team used α-pinene, a molecule that helps to give fir
forests their characteristic smell, but compounds from other types of
vegetation might show a similar effect, the scientists say.

In the third paper, published in Science,
a team that includes some of Kirkby’s co-authors reported a similar
finding using a different experiment. Federico Bianchi, a chemist now at
the University of Helsinki, and his collaborators measured the
composition of air and monitored the weather at the Jungfraujoch
Research Station in the Swiss Alps at around 3,500 metres in altitude.
They found that molecules similar to α-pinene that could also originate
from vegetation can seed clouds without much sulphuric acid.

In addition to feeding into climate
predictions, the findings have another potential implication, says
atmospheric scientist Bjorn Stevens of the Max Planck Institute for
Meteorology in Hamburg, Germany. Some scientists have warned that
measures such as scrubbing sulphur dioxide from coal-plant emissions
could remove some of the beneficial cooling effect of clouds and boost
global warming, but this may now be less of a concern because trees can
seed clouds too. “What it means is, we don’t have to fear clean air,”
says Stevens.
It is also interesting to
speculate whether trees emit these compounds in part because there is a
benefit to them in making their own climate, Kirkby says. “This really
does touch on the Gaia hypothesis,” he says, referring to the theory
that Earth’s life behaves as a single organism that tends to preserve
itself. “It’s a beautiful mechanism for trees to control their environment.”